Low-power image change detector

ABSTRACT

A sensing device projects near-field spatial modulations onto a closely spaced photodetector array. Due to physical properties of the grating, the point-spread response distributes spatial modulations over a relatively large area on the array. The spatial modulations are captured by the array, and photographs and other image information can be extracted from the resultant data. An image-change detector incorporating such a sensing device uses very little power because only a small number of active pixels are required to cover a visual field.

BACKGROUND

Traditional cameras use a lens or lenses to image each point in a scene onto a single point on a sensor. In digital cameras, the sensor is a two-dimensional array of picture elements, or “pixels,” that encodes the imaged scene into digital image data for storage, processing, and reproduction.

Digital imaging has enabled new imaging architectures. Cathey and Dowski took an early and conceptually important step away from the traditional model by exploiting digital processing. They designed a cubic-phase optical plate which, when inserted into the optical path of a traditional camera, led to an image whose (significant) blur was independent of the object depth: the image on the sensor plane did not “look good” as it would in a traditional camera. However, subsequent image processing sharpened the entire blurred image, thus leading to enhanced depth of field. Since then the field of computational imaging has explored imaging architectures in which the raw signals do not superficially resemble a traditional image; instead, the final image is computed from such signals. More and more of the total imaging “burden” is borne by computation, thereby expanding the class of usable optical components. In this way, many optical aberrations can be corrected computationally rather than optically. This imaging paradigm has led to new conceptual foundations of joint design of optics and image processing, as well as a wide range of non-standard imaging architectures such as plenoptic, coded-aperture and multi-aperture systems, each with associated methods of signal processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:

FIG. 1A is a cut-away view of a sensing device 100 with an odd-symmetry grating 105 overlying a photodetector array 110, such as a CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) sensor.

FIG. 1B depicts sensor 100 of FIG. 1A simulating light incident plane 120 at an acute angle 160 to illustrate the sensitivity of curtains 140 and foci 145 to the angle of incidence.

FIG. 2 depicts a binary odd-symmetry grating 200 in accordance with one embodiment.

FIG. 3 depicts a sensing device 300 in accordance with an embodiment in which a binary, odd-symmetry phase grating 310 is formed by an interface between materials of two different refractive indices.

FIG. 4A is a plan view of a sensor 400 in accordance with another embodiment.

FIG. 4B is a three-dimensional perspective of sensor 400 of FIG. 4A, and shows how light 420 from a direction normal to the grating surface casts an interference pattern 425 on an underlying photodiode array 430.

FIGS. 5A, 5B, 5C, and 5D each depict three boundaries of odd symmetry 500 over a two-dimensional photodiode array 505.

FIG. 6 depicts three odd-symmetry gratings 600, 620, and 630, each with feature segments of different relative widths.

FIG. 7A is a cross-section of a phase grating 700 in accordance with an embodiment that uses more than two levels to produce an odd symmetry.

FIG. 7B is a cross-section of a phase grating 710 that is optically similar to phase grating 700 of FIG. 7A, but uses fewer layers.

FIG. 8 is a cross-section of a phase grating 800 that illustrates how odd symmetry can be extended to curved functions.

FIG. 9 is a plan view of a grating 900 in accordance with an embodiment in which boundaries of odd symmetry 905 extend radially from the center of the grating, and in which the widths of the feature segments widen gradually away from the center.

FIG. 10 is a plan view of a grating 1000 in accordance with an embodiment with concentric boundaries of odd symmetry 1005, and includes a cut-away view along line A-A.

FIG. 11 is a plan view of a grating 1100 in accordance with an embodiment similar to grating 900 of FIG. 9.

FIG. 12 is a plan view of a grating 1200 in accordance with another embodiment.

FIG. 13 depicts a grating 1300 in accordance with another embodiment.

FIG. 14 depicts a grating 1400 and associated photodiode array 1405.

FIG. 15 depicts a grating 1500 and associated photodiode array 1505.

FIG. 16 is a plan view of a grating 1600 in accordance with an embodiment with pentagonal boundaries of odd symmetry 1605.

FIG. 17A is a plan view of a grating 1700 in accordance with another embodiment.

FIG. 17B depicts the shapes of boundaries 1705 of FIG. 17A.

FIG. 18 depicts a two-dimensional array 1800 of gratings 1805 disposed over a photodiode array (not shown).

FIG. 19 is a flowchart 1900 detailing how an image 1905 is captured and resolved in accordance with grating 1700 of FIG. 17.

FIG. 20 depicts lithographic process for forming an image sensor 2000 in accordance with one embodiment.

FIG. 21A depicts a camera 2100 in accordance with an embodiment that includes a a lens 2105.

FIG. 21B is an example of camera 2100 with a point source 2125 imaged in focus on array 2115.

FIG. 21C is an example of camera 2100 with a point source 2140 imaged out of focus on array 2115.

FIG. 21D is an example of camera 2100 with a point source 2155 imaged more out of focus than point source 2140 in the example of FIG. 21C.

FIG. 22 is a plan view of a portion of an array of pixels 2200 illuminated with three PSFs 2205, 2210, and 2215.

FIG. 23 depicts three spiral PSFs 2300, 2305, and 2310 to illustrate how cameras in accordance with some embodiments can compensate for lens aberrations, including spherical aberration, coma, and Petzval field curvature.

FIG. 24 depicts a tessellated optical element 2400 to illustrate aspects of phase gratings in accordance with some embodiments.

FIG. 25 depicts how the rich pattern of spatial modulations within orientation chirps produced by grating 2400 of FIG. 24 facilitates improved resolution for out-of-focus PSFs.

FIGS. 26A and 26B depict tessellated gratings 2600 and 2605 in accordance with some embodiments.

FIGS. 27A and 27B depict tessellated gratings 2700 and 2705 in accordance with some embodiments.

FIG. 28 depicts a tessellated grating 2800 in accordance with one embodiment.

FIG. 29 depicts a tessellated grating 2900 in accordance with another embodiment.

FIG. 30 depicts a filter array 3000 that can be used in accordance with some embodiments to produce color images using cameras of the type detailed in FIGS. 21A-D.

FIG. 31 depicts a color channel 3100, one of four color channels for the embodiment introduced in connection with FIG. 30.

FIG. 32 depicts an image-change detector 3200 that supports a low-power mode.

FIG. 33 depicts array 3205 of FIG. 32 as an array of pixels 3300.

FIG. 34 depicts array 3205 of FIG. 33 with twenty-five nonadjacent pixels 3300 darkened to indicate a subset n2 that is active in an example of a tentative mode in which an increased number of pixels are polled for increased motion sensitivity.

FIG. 35 depicts response 1930 of FIG. 19 as an exemplary chirp that defines a convex hull 3500.

FIG. 36 is a flowchart 3600 illustrating a method of detecting apparent motion in accordance with one embodiment of detector 3200 of FIG. 32.

DETAILED DESCRIPTION

The economic pressures for miniaturization of electronic devices, including cameras, arising in the mobile computing market have led to smaller imager form factors. One technique for miniaturizing imaging architectures is one based on integrating diffractive optics with photodetector arrays. Such architectures can forgo lenses and rely instead on diffraction gratings that can be created using processes similar to those used to create the underlying sensor. For a given image resolution, such diffractive elements enable the construction of imaging devices much smaller than possible using the optical paths of traditional cameras, and at a much lower cost.

FIG. 1A is a cut-away view of a sensing device 100 with integrated diffractive optics. A grating 105 overlies a photodetector array 110, such as a CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) sensor. The features of grating 105 offer considerable insensitivity to the wavelength of incident light in a wavelength band of interest, and also to the manufactured distance between grating 105 and photodetector array 110. Grating 105 produces an interference pattern for capture by array 110. Photographs and other image information can then be extracted from the pattern.

Light in a wavelength band of interest—such as the visible spectrum—is incident grating 105 from a direction 115 that is normal to a transverse plane 120 of the grating 105. Dashed lines 125 highlight periodic boundaries of substantially odd symmetry. Each of these boundaries is a result of features 130 and 135 of odd symmetry, and produces a normally arranged curtain 140 of minimum intensity created by destructive phase interference between adjacent features 130 and 135. Curtains 140 are separated by foci 145, and the collection of curtains 140 and foci 145 (curtains of maximum light intensity) extend from grating 105 through the body 150 of device 100 to produce an interference pattern on photodetector array 110. In this illustration, the pattern of intensity variations evident in the foci and curtains are near-field spatial modulations that result from near-field diffraction. One photosensitive element 155 within array 110 is shaded beneath a focus 145 to serve as a reference for a subsequent discussion of the sensitivity of device 100 to the angle of incident light.

The image of FIG. 1A resulted from a simulation of a sensing device with the following parameters and assuming specific parameters. Body 150 is of fused silica, and is in contact with a conventional photodetector array 110 with photosensitive elements spaced by 2.2 μm. The top of grating 105 is an air interface in this example. The relatively small segments of features 130 and 135 are about 1 μm, and the relatively larger segments are about 4 μm. These segments generally form transverse plane 120, which is separate from array 110 by about 25 μm. Curtains 140 and foci 145 are the destructive and constructive interference patterns for 532 nm incident light.

The thickness of body 150 and lengths of the segments of features 130 and 135 were optimized for 400 nm light despite the selection of 532 nm light for the simulation. As a consequence, the tightest focus occurs about 5 μm above array 110 (at the 20 μm mark). The resultant curtains 140 plainly separate foci 145 well above and below the 20 μm mark, however, illustrating a robust insensitivity to wavelength within the band of interest. The relatively deep and continuous penetration of curtains 140 also provides considerable manufacturing tolerance for the thickness of body 150. These advantages obtain because the near-field spatial modulations projected onto array 110 are wavelength independent over the wavelength band of interest, which means that the adjacent modulations (dark and light) do not reverse signs with changes in wavelength within the band of interest.

FIG. 1B depicts sensor 100 of FIG. 1A simulating light incident plane 120 at an acute angle 160 to illustrate the sensitivity of curtains 140 and foci 145 to the angle of incidence. Using element 155 as a reference point, we see that that the foci 145 that illuminated element 155 in FIG. 1A has considerably moved to the right in FIG. 1B. Curtains 140 and foci 145 extend at an acute angle that relates to angle 160 according to Snell's law. The separation of foci 145 by curtains 140 is maintained. Sensor 100 is thus sensitive to the angle of incidence.

FIG. 2 depicts a binary odd-symmetry grating 200 in accordance with one embodiment. Each of three boundaries of odd symmetry is indicated using a vertical, dashed line. The upper features of grating 200 are at a height sufficient to induce one half wavelength of retardation in the band of interest relative to lower features, or π radians of relative phase delay. Features 205 and 210 on either side of each boundary exhibit odd symmetry with three differently sized segments W₀, W₁, and W₂. With this arrangement, paired segments (e.g., W₀ within features 205 and 210) induce respective phase delays that differ by approximately half a wavelength over the wavelength band of interest.

FIG. 3 depicts a sensing device 300 in accordance with an embodiment in which a binary, odd-symmetry phase grating 310 is formed by an interface between materials of two different refractive indices, a polycarbonate layer 315 and optical lanthanum dense flint glass 320 in this example. Each of four boundaries of odd symmetry 325 is indicated using a vertical, dashed line. As in the foregoing examples, the upper features of grating 310 induce phase retardations of half of one wavelength (π radians) relative to lower features. Features 330 and 335 on either side of each boundary exhibit odd symmetry. With this arrangement, paired features induce respective phase delays that differ by approximately half a wavelength over the wavelength band of interest.

These elements produce an interference pattern on an analyzer layer 327 (e.g., a conventional photodiode array) in the manner detailed in connection with FIGS. 1A and 1B. This example assumes light incident the light interface of grating 300 is normal to the transverse plane of phase grating 310, in which case light fields that enter grating 310 equidistant from a one of the boundaries of odd symmetry 325, such as at locations (−X,0) and (X,0), are out of phase at points beneath array 310 (e.g., point (0,Z)), and thus destructively interfere to produce curtains of minimum intensity (e.g., curtains 140 of FIG. 1). Neither the depth Z nor the wavelength of light over a substantial spectrum significantly influences this destructive interference. Constructive interference similarly produces foci of maximum intensity (e.g., foci 145 of FIG. 1). Both the high and low features admit light, which provides relatively high quantum efficiency relative to gratings that selectively block light.

The following discussion details phase gratings in accordance with examples described by Patrick R. Gill and David G. Stork in an upcoming paper. “Lensless Ultra-Miniature Images Using Odd-Symmetry Spiral Phase Gratings.” © 2013 Optical Society of America. In that article, Gill and Stork describe a phase grating formed by a high-n, low-dispersion substrate and a low-n, high-dispersion coating that can introduce approximately independent phase shifts in all normally incident visible light. Similar gratings are discussed above. If there exist certain points p on this interface that satisfy the following symmetry in their transmission t(·) and phase retardation ϕ(·), t(p+y)=t(p−y)∀y  (1) ϕ(p+y)=ϕ(p−y)+π°2nπ∀y,nϵI  (2) where y is a horizontal translation transverse to the grating direction, then the grating has odd symmetry about points p, and light will interfere destructively below p, regardless of λ and depth z.

A linear odd-symmetry grating above a photosensor array could pass information from a single spatial orientation of features in the far field (transverse to the grating orientation). However, to capture information about arbitrarily oriented features of a complex scene, it is preferable to have a complete distribution of orientations in the diffractive optic. More generally, if the point-source responses (PSRs) are approximately spatially invariant, the transfer function of the imager approximates convolution with the PSR function. In such a case, the PSR should have significant power at all 2D spatial frequencies to make the inversion problem of image recovery well-conditioned.

In one example provided in Gill and Stork, gratings were numerically optimized to focus visible light onto a photodetector array 100 μm below. Optical simulations estimated the imaging performance of such a device from a 60×60 pixel array with 2.2 μm pitch 100 μm below the gratings with the sensor illuminated by a complex scene far (>>100 μm) from the sensor. The resultant photocurrent from the pixel array was unintelligible; however, the scene was reconstructed to a higher resolution than possible using a much larger diffractive imagers based on Talbot-effect angle-sensitive using Tikhonov regularization. Gill and Stork report that compressed sensing techniques could be applied to improve the reconstruction quality if the scene is known to have a compressible structure. Compressed sensing could be especially advantageous if small gaps in the Fourier transform of the PSR exist.

FIG. 4A is a plan view of a sensor 400 in accordance with another embodiment. Relatively high segments 405 and low segments 410 on either side of each of eight boundaries of odd symmetry 415 create a grating in which the widths of the segments increase with distance from the center of the sensor. For a given focal depth, light of higher frequencies tends to produce a sharper focus with narrower feature widths. Sensor 400 can therefore be optimized such that the central portion of the grating is optimized for collection of relatively higher frequency light, and the peripheral area for collection of relatively lower frequency light. This topic is detailed below in connection with other Figures.

FIG. 4B is a three-dimensional perspective of sensor 400 of FIG. 4A, and shows how light 420 from a direction normal to the grating surface casts an interference pattern 425 on an underlying photodiode array 430. Curtains and foci, as detailed previously, respectively cast shadows 435 and bright shapes 440 to be sensed by individual photosensitive elements 445 of array 430. Array 430 captures a digital representation of pattern 425.

FIGS. 5A, 5B, 5C, and 5D each depict three boundaries of odd symmetry 500 over a two-dimensional photodiode array 505. Curtains 510 cast shadows 515 on the underlying photodetectors 520, and the patterns thus created are different depending upon the angle of incident light. Array 505 can therefore sample the resultant interference pattern to obtain information as to the angle of incidence.

FIG. 6 depicts three odd-symmetry gratings 600, 620, and 630, each with feature segments of different relative widths. It can be useful to create a sensor with multiple width ratios, as shown, to compensate for manufacturing tolerances that impact the relative heights of the grating features. Assuming, for example, that grating 600 is width optimized for a manufacturing process of interest, but that the process produces a relative phase delay of 40% rather than the ideal 50% to form curtains of minimum intensity at the desired positions. To a first order the increased width of the relatively wide segments, as depicted in grating 630, can improve the distortion resulting from the erroneous phase offset. Phase offsets above 50% can be corrected for by narrowing the relatively wide segments, as depicted in grating 620. Some embodiments include a mixture of relative segment widths covering different areas of a photodiode array to accommodate manufacturing tolerances. Images associated with the gratings that provide the sharpest focus, or the sharpest focus for a wavelength of range of wavelengths, can be selected or combined to obtain the desired image data. The different gratings may also perform better for light of different wavelengths or incident angles, so selection of which gratings to use for a given image may be optimized for variables other than manufacturing tolerances.

FIG. 7A is a cross-section of a phase grating 700 in accordance with an embodiment that uses more than two levels to produce an odd symmetry. Additional levels may allow for sharper focus, but may require more complex manufacturing processes. If gratings are to be made using photolithography, for example, additional levels require additional mask steps. Paired surfaces on either side of each boundary of odd symmetry introduce respective paired phase delays that differ by approximately half a wavelength, plus an integer number of wavelengths, over the wavelength band of interest.

FIG. 7B is a cross-section of a phase grating 710 that is optically similar to phase grating 700 of FIG. 7A, but uses fewer layers. The resultant larger abrupt discontinuities 715 may introduce undesirable image artifacts or may be difficult to manufacture accurately, but the reduced number of levels may reduce manufacturing costs.

FIG. 8 is a cross-section of a phase grating 800 that illustrates how odd symmetry can be extended to curved functions.

FIG. 9 is a plan view of a grating 900 in accordance with an embodiment in which boundaries of odd symmetry 905 extend radially from the center of the grating, and in which the widths of the feature segments widen gradually away from the center. Grating 900 captures image information at sixteen discreet angles with a continuously variable set of widths. While convenient to draw grating 900 as a circle, other shapes may be used. In some embodiments, for example, collections of gratings are arrayed over a photodiode array. In such cases grids that share common boundaries (e.g., such as hexagonal, square, or triangular boundaries) make more efficient use of the underlying photodiodes.

FIG. 10 is a plan view of a grating 1000 in accordance with an embodiment with concentric boundaries of substantially odd symmetry 1005, and includes a cut-away view along line A-A. In this example the widths of the feature segments are discrete and the angles are continuous. The spacings of grating 1000 appear consistent, but may be varied to allow for sharp focus for a range of wavelengths, angles of incidence, or manufacturing variations.

FIG. 11 is a plan view of a grating 1100 in accordance with an embodiment similar to grating 900 of FIG. 9. The two halves of grating 900 provide essentially the same information. Grating 1100 adds half-circle polarization filters 1105 and 1110 with perpendicular orientations. Each half of grating 1100 thus produces image data specific to one of two polarizations, and these data can be used separately or together. More or fewer filters, with the same or different orientations, may be used in other embodiments. Different types of filters can also be used to cover all or a portion of gratings of the type described herein.

FIG. 12 is a plan view of a grating 1200 in accordance with another embodiment. Curved boundaries of odd symmetry 1205 extend radially from the center of the grating, and the widths of the feature segments widen gradually away from the center. The curvature of boundaries 1205 provide continuously varying angular information similar to what is available from grating 1000 of FIG. 10 while retaining the continuously varying spacings of grating 900 of FIG. 9.

FIG. 13 depicts a grating 1300 in accordance with another embodiment. As noted previously, different widths of the grating features provide sharper focus for different colors of light within the wavelength band of interest. Grating 1300 has the same radial symmetry of grating 900 of FIG. 9, but those areas for which the spacing is optimized for blue, green, and red light are provided with filters to admit their respective wavelengths. Omitting wavelengths that provide a blurred interference pattern on the underlying analyzer can improve image sharpness, and can allow more accurate reconstruction of color image data. Grating 1300 is bounded by an opaque mask 1305 that defines the limit of the aperture.

FIG. 14 depicts a grating 1400 and associated photodiode array 1405. Grating 1400 has parallel odd-symmetry boundaries 1410, which may have features of the same or different widths, or of varying widths along one or more boundaries. Parallel boundaries with the requisite diversity of widths and spacings to sample a sufficient number of spatial frequencies can image one-dimensional images, e.g., barcodes. Array 1405 is shown alongside, rather than below, grating 1400 to highlight the angle θ_(A) between the direction of boundaries 1410 and the columns of photosensitive elements in array 1405. Angle θ_(A) creates more diversity of measurements because the linear shadow covers different percentages of pixels in different rows. In one embodiment angle θ_(A) is selected so that the top of each boundary is offset from the bottom by about one pixel of array 1405.

FIG. 15 depicts a grating 1500 and associated photodiode array 1505. Grating 1500 has parallel, right-angled boundaries 1510, which may have features of the same or different widths, or of varying widths along one or more boundaries. Parallel boundaries with the requisite diversity of widths and spacings along two dimensions to sample a sufficient number of spatial frequencies can image e.g. point sources, such as to identify the position of the sun, a fiducial LED, or a retroreflective element or patch used for motion capture. Angle θ_(A) can be introduced for the reasons presented above in connection with FIG. 14. Point source identification may also be accomplished with a grating that is also suitable for an imaging function.

FIG. 16 is a plan view of a grating 1600 in accordance with an embodiment with pentagonal boundaries of odd symmetry 1605. In this example the widths of the feature segments are discrete, but they can vary along one or more boundaries in other embodiments. Straight boundaries may be advantageous over curved ones because line segments can more easily provide precise odd symmetry.

Grating 1600 provides information at five different orientations. Other boundary shapes, such as other polygons, are used in other embodiments. In general, polygons with odd numbers of sides provide greater orientation diversity than polygons with a similar but even number of sides (e.g., a pentagon provides more orientation diversity than a square or a hexagon).

FIG. 17A is a plan view of a grating 1700 in accordance with another embodiment. Recalling that relatively narrow (wide) segment spacing works better for relatively high (low) frequencies, feature spacing increases along odd-symmetry boundaries (between dark and light regions) with distance from the center. Curved boundaries of odd symmetry 1705 extend radially from the center of the grating to the periphery, radiating out between the dark (elevated) and light (recessed) arms near the center. The curved boundaries are obscured by grating features in FIG. 17A, so the shapes of boundaries 1705 are depicted in FIG. 17B for ease of review.

The segment widths do not continue to increase with radius, as there is a maximum desired width for a given wavelength band of interest (e.g., the widest may correspond to the lowest frequency of visible red light). The features that define boundaries 1705 therefore exhibit discontinuities as they extend toward the periphery of grating 1700. In this example, grating 1700 has three discrete areas each tuned to a subset or all of the wavelengths in the band of interest.

FIG. 18 depicts a two-dimensional array 1800 of gratings 1805 disposed over a photodiode array (not shown). Each of gratings 1805 is identical, but any number of parameters, many of which are discussed previously, can be varied within and among gratings 1805. For example, different shapes and types of gratings can be used to create and image different types of interference patterns that can be combined or used separately to obtain some desired result. The decision to consider all or a specific subset of information generated by one or more of the constituent gratings can be done once, such as at time of manufacture to accommodate process variations, or can be done dynamically to highlight different aspects of a scene. Emphasizing aspects of different patterns can be used, for example, to highlight light of different polarizations, wavelengths, or angles of incidence.

Spaced gratings facing the same direction, particularly when their characteristics are well matched, can be used to sense moving objects. Assuming matched gratings with a fixed separation receiving light from the same scene, the difference between the photocurrents of the respective analyzer layers is sensitive only to objects relatively close to the pair. Further, the time derivative of this difference is sensitive to nearby, moving objects, and is insensitive to relatively distant moving or stationary objects.

FIG. 19 is a flowchart 1900 detailing how an image is captured and resolved in accordance with grating 1700 of FIG. 17. First, an image 1910 is presented such that light from image 1910 is incident grating 1700. The incident light passes through phase grating 1700 to produce an intensity pattern 1920 on an underlying two-dimensional array of photosensors (not shown), which captures the pattern (1915). The captured pattern 1920 may appear unintelligible to a human; however, because grating 1700 has sharp features in its point-spread function (PSF), the pattern contains rich information about the image.

The PSF of grating 1700, possibly in combination with the underlying array, is known from a prior calibration or high-fidelity simulation. The way in which the PSF varies as a function of incident angle and color may also be similarly determined. This information is represented by a response 1930. A mathematical conversion based on this response can thus be used to reconstruct image 1910 from pattern 1920.

To recover the original image, responses 1920 and 1930 are combined to form an inverse problem (1925), which is solved (1935) to recover a version 1940 of the original image. One embodiment employs the well-known Tikhonov regularized inversion technique to accomplish steps 1925 and 1935. Take as a starting point a) detailed knowledge of the PSF of grating 1700, b) knowledge of the noise level of the system under current illumination conditions, and c) the specific readings observed for this image (pattern 1920). Express the unknown image as an N×1 vector x, where N is the total number of pixels one wishes to reconstruct. Express the readings from the photosensor as an M×1 vector y, where M is the total number of photosensors in the array. Express detailed knowledge of the PSF as an M×N matrix A such that for any image x, the formula yielding expected observed signal y under x is y=Ax, called the “forward equation.”

To reconstruct an image, it suffices to solve the forward equation with a known measurement vector y for an unknown image x as follows. Multiply both sides of the forward equation by A^(T) (the transpose of A) to obtain A^(T) y=A^(T) Ax. The matrix A^(T) A is square and in principle could be directly inverted to recover x; however usually this inversion is poorly conditioned when noise is present and when not all eigenvectors of A^(T) A have equally large associated eigenvalues. Thus in practice, Tikhonov regularization (as follows) usually delivers preferable results.

Next, select a regularization parameter λ>0 based on the noise level at the current illumination conditions. Finally, invert the matrix (A^(T) A+λI) (where I is the identity matrix), assume (A^(T) A+λI)≈(A^(T) A) and multiply on the left of the preceding equation to obtain: x≈(A ^(T) A+λI)⁻¹ A ^(T) y  (3) Therefore, for a given regularization parameter λ, the image recovered through Tikhonov regularization is a linear combination of the readings from the photosensor. If the PSF is sufficiently spatially invariant to the extent that its spatial dependence can be neglected, these computations can be done in the Fourier domain, allowing for much faster numerics.

Another embodiment recovers the matrix x using compressed sensing. If the scene is expected to be sparse in some basis (such as a wavelet transform W for natural images), the following methodology can be used. We can recover the sparse scene components z where x=Wz by finding the z that minimizes the following cost function: ½ r^(T)r+λf(z), where r is the residual (y−AWz), λ>0 is a regularization parameter (different from that used in Tikhonov regularization, but also noise-dependent), and f(z) is a function penalizing non-sparse z. If f(z) is a convex function of z such as the L₁ norm, this optimization problem can be solved efficiently using convex optimization techniques. The penalty function f(z) can also take on other forms, including terms penalizing total variation in the reconstructed image x or other prior scene knowledge.

Some of the chief advantages of compressed sensing over linear approaches such as Tikhonov regularization are that the former allow more prior information about the expected scene structure to help shape the final image. Further, if A^(T) A does not have full rank or cannot measure certain aspects of the scene (for example, due to some near-zero regions of the 2D Fourier transform of the PSF), using compressed sensing sometimes overcomes these limitations given correct prior information about the expected images.

The foregoing Tikhonov and compressed-sensing techniques can include iterative methods to reduce problem complexity. For example, Richardson-Lucy deconvolution can iteratively approximate Tikhonov regularized inversion and iterated wavelet thresholding can be a numerically efficient way to converge to a compressed-sensing-like solution.

In some embodiments the purpose of the sensor is not to reconstruct an image, but to perform some optical sensing task. In such cases the vector x may represent the sought measurement rather than the field of image pixels, and the forward transform A can be appropriately modified.

FIG. 20 depicts a lithographic process for forming an image sensor 2000 in accordance with one embodiment. First, a wafer 2005 of material that is transparent over the wavelength band of interest is patterned with a mask 2010 that defines the relatively high features of what will become an odd-symmetry grating surface of the type detailed herein. Next, the exposed surface of wafer 2005 is etched to create recessed regions 2015. Mask 2010 is then removed. Finally, wafer 2005, now comprising a grating, is bonded to a photodiode array 2025. Photolithographic and wafer-bonding processes are well known to those of skill in the art, so a detailed discussion is omitted.

FIG. 21A depicts a camera 2100 in accordance with an embodiment in which a converging optical element, in this case a lens 2105 (although a single-element lens is shown for simplicity of illustration, generally the optical element can comprise one or more refractive, diffractive, and/or reflective elements), is used in conjunction with a phase grating element, grating 2110, disposed in the path between the optical element and a dense photodetector array 2115 to form images thereon. A scene incident the front side of lens 2105 is projected through grating 2110 and onto array 2115. Grating 2110 induces spatial modulations in the incoming light and passes the resulting interference pattern to array 2115, which captures a digital representation of the spatial modulations. An integrated processor 2120 electrically coupled to array 2115 computes an image of the scene from the digital representation. The processor is shown also physically coupled to array 2115, but the processor can be located elsewhere in other embodiments.

Lens 2105 defines a front focal point FFP and a rear focal point RFP, and is spaced from grating 2110 by a distance less than the image-plane distance D between lens 2105 and focal point RFP. Array 2115 is on the opposite side of focal point RFP from grating 2110 in this example. Grating 2110 may be an odd-symmetry grating that has properties detailed above in connection with the preceding figures. In other embodiments (such as an embodiment primarily operating in a macro mode) the focal length of lens 2105, defined for objects at infinity, may be closer to lens 2105 than to grating 2110, or may move over a range that encompasses such relative positioning.

Surface features of grating 2110 are separated from array 2115 by a distance X. Though shown as separate structures for ease of illustration, grating 2110 can be integrated with or attached to array 2115. Distance X in camera 2100 is, in this example, no more than 400 times a longest wavelength of interest λ_(max) in the medium(s) between the surface features of grating 2110 and array 2115 (X≤400λ_(max)). For example, a camera in which λ_(max) is 0.5 microns may have a spacing X between the features of grating 2110 and the surface of array 2115 of up to 200 microns.

FIG. 21B is an example of camera 2100 with a point source 2125, represented by tip of an arrow, that is imaged in focus on array 2115. Grating 2110 is out of the focal plane, so the light from lens 2105 presents a blur-spot PSF 2130 to grating 2110. (As in other examples used herein, the area occupied by PSF 2130 refers to the area of the central lobe.) Grating 2110 produces an interference pattern from function 2130, but the illumination boundaries of the pattern are not evident in the tightly focused, diffraction-limited spot 2135 on array 2115. Objects at the range and position of point source 2125 are tightly focused (field curvature and other aberrations may change the best focus range for other positions), and are nominally imaged at the full resolution of array 2115, assuming lens 2105 is capable of such resolution.

FIG. 21C is an example of camera 2100 with a point source 2140 that is imaged out of focus on array 2115. As in the prior example, the light from lens 2105 presents a blur-spot PSF 2145 to grating 2110, and grating 2110 produces a pattern of spatial modulations. Because point source 2140 is imaged out of focus, however, the area of PSF 2150 at array 2115 is greater than in the example of FIG. 21B, and illumination transitions/substructure within the pattern area are evident. In camera 2100, these illumination transitions are near-field spatial modulations induced by features of grating 2110. The resultant spiral pattern of PSF 2150 is preferably an invertible orientation chirp. As used herein, an “orientation chirp” is a pattern of spatial modulations that cover ranges of spatial frequencies and orientations sufficient to recover an image at a desired resolution.

FIG. 21D is an example of camera 2100 with a point source 2155 that is imaged more out of focus than point source 2140 in the example of FIG. 21C. Light from lens 2105 presents a blur-spot PSF 2160 that is still greater than PSF 2145, and a resultant invertible PSF 2165 on array 2115 is similarly larger than PSF 2150. Although not shown, imaging a point source at the FFP of FIG. 21A produces an invertible PSF including orientation chirp features. Two point sources, one in front of and one behind point 2125 but along the same optical axis, may produce similar-sized orientation chirps. Due to aberrations in the lens system, however, the chirps may differ—such differing characteristics may be used to resolve range, as detailed further below.

FIGS. 21A-D illustrate the general point that the pattern area and the richness of the accompanying spatial modulations on array 2115 are a function of focus, the duller the focus the greater the area and the better resolved the spatial modulations. Point sources farther away from lens 2105 than point source 2125 of FIG. 21A produce ever larger PSFs on the array as they move away from (or towards) lens 2105.

The PSF for an out-of-focus point source is a scaled version of an orientation chirp from grating 2110, where the diameter of the orientation chirp is proportional to defocus of the point source. The observations at the sensor plane (the surface of array 2115) will therefore be the in-focus and out-of-focus imaged points, each convolved with the orientation chirp at a chirp phase dependent upon the position the light ray bundle received from that point, scaled according to an out-of-focus parameter, and spatially superimposed with like contributions from other imaged points. Camera 2100 can recover relatively high-resolution images of out-of-focus objects because this convolution is computationally invertible for the majority of common image capture situations. In this context, “computationally invertible” means that image data can be recovered to a specified degree of precision using e.g. inverse, pseudoinverse, and compressed-sensing transformations. A PSF is computationally invertible, for example, if its 2D Fourier transform is “complete,” or has substantial amplitude at all spatial frequencies required to recover an image at a specified resolution.

Not all spiral PSFs are complete. For example, Archimedean spirals have regularly spaced arms whose Fourier transforms have peaks at the reciprocal of the inter-arm period and nulls between these peaks. In contrast, the spiral PSR 1930 of FIG. 19 has few, unevenly spaced arms that are sharply bounded and sweep through all orientations, so it has significant Fourier power at all spatial frequencies and is complete. Due to this completeness, accurate deconvolution is relatively well-conditioned, so undoing the effect of the PSF is relatively straightforward. Regardless of whether computations are performed in the Fourier domain or the spatial domain, deconvolution works well if the Fourier transform of the PSF has no zeros. In the case that a point source causes a blur spot 2160 that is not concentric with a spiral, the resulting PSF will contain a spatially wrapped version of the PSF. Spatially wrapping the spiral does not substantially affect its completeness.

Camera 2100 can measure light intensity from photodetector array 2115 without first needing to focus (although some embodiments can focus manually or automatically). Data captured by array 2115 includes orientation chirps with Fourier-component strengths that vary with depth (see FIGS. 21B-D). The Fourier transform of the local observations will be the product of the imaged object's Fourier transform and the depth-dependent Fourier transform of the orientation chirp. By searching for the depth-specific kernel that best matches this product for each imaged point, scene depth can be determined, assuming the scene has some texture, as detailed below.

The depth d of a local scene patch x can be inferred from readings y through Bayesian estimation as follows. First, a likelihood p(y|d) of each depth can be computed by a further Bayesian estimation based on knowledge that the Fourier transform of y is the product of the Fourier transforms of x and the depth-dependent PSF, and with knowledge of typical power spectra of photographed objects. Next, this likelihood p(y|d) is weighted by a Bayesian prior on the known distribution of depths and depth changes in a scene to arrive at a posterior probability of p(d|x) for depth at each point in the scene. Bayesian estimation of the depth map of a scene based on depth and depth change priors, as well as point-wise estimates of depth associated with corresponding certainty (indicated by the height of peaks in the likelihood p(y|d)) is a technique known to those skilled in the art, and will not be further discussed here. In this application, knowledge of the true depth map is important for accurate image recovery (to be described shortly) precisely for those images that have significant Fourier power in spatial frequencies that interact with the Fourier transform of the PSF. Thus, accurate depth maps are possible where the scene has fine texture, and where scene patches lack this texture convolution with the PSF does not degrade image quality in the scene.

Next, the Fourier transforms are deconvolved in image space or the Fourier domain; the problem scale will dictate which of these is faster. The deconvolution kernel can also be made to vary with light level for a Weiner-optimal reconstruction (although humans tend to prefer overly-sharpened images; this sharpening filter can be incorporated with the deconvolution filter to save an additional step).

The result of selecting the correct filter followed by deconvolution is a depth map and a reconstruction of the original image. If the orientation chirp is Fourier-complete, the reconstructed image can resolve the same number of pixels as array 2115. This is unlike most plenoptic cameras, and is made possible by the fact that each pixel reading contributes useful information to the deconvolution problem. In the case where a PSF's high-frequency components are small, processor 2120 may smooth the highest spatial frequencies to avoid adding too much noise. In low-light conditions, camera 2100 may lose e.g. a factor of two in resolved pixels due to this effect; this represents an improvement over existing plenoptic cameras, whose pixel efficiency may be as low as 4%. For well-formed orientation chirps according to an embodiment and general imaging conditions, PSFs with a central lobe diameter up to six photodetector pitches should be invertible to recover image features with a spatial frequency up to at least 0.25 cycles per photodetector (Nyquist frequency being 0.5 cycles per photodetector pitch in the major dimensions of the photodetector array). Such performance depends in part on the lens element having a sufficient modulation transfer function at the relevant spatial frequencies.

FIG. 22 is a plan view of a portion of an array of pixels 2200 illuminated with three PSFs 2205, 2210, and 2215. PSF 2205 is an orientation chirp representing a sharply focused point source; illumination substructure cannot be resolved given the pitch of array 2200. If all points of a given scene are in focus, image resolution is primarily a function of array pitch, or of array pitch and the diameter of a diffraction-limited spot.

PSF 2210 is an orientation chip representing a poorly focused point source; spatial modulations appear as spiral arms of a computationally rich PSF that can be resolved by array 2200 to locate the corresponding point source in the image. Finally, PSF 2215 represents a point source whose focus is between those of PSFs 2205 and 2215; spatial modulations can again be resolved to locate the corresponding point source in the image.

For both PSF 2210 and 2215, the resolution of the image is limited by the larger of the pitch and the spacing of the separation between arms of the PSF spiral. In this illustration, the three point sources are easily located in the two dimensions of array 2200. Further, the three disparate pattern areas of the three PSFs provide a measure of distance in a dimension normal to array 2200. Cameras like camera 2100 of FIGS. 21A-D can therefore provide extended depths of field, focused images for out-of-focus objects, and measures of distance from image data.

FIG. 23 depicts three spiral PSFs 2300, 2305, and 2310 to illustrate how cameras in accordance with some embodiments can compensate for lens aberrations, including spherical aberration, coma, and Petzval field curvature. Such compensation can simplify primary lens design and allow an increase in aperture without sacrificing image quality.

Spherical aberration is the condition whereby the focal length of a given annulus of a lens varies linearly with the annulus' radius. In the configuration of FIG. 21, this condition may influence the shapes of orientation chirps on the array. PSF 2300 of FIG. 23 is a hypothetical ideal chirp, the result of a perfect lens. PSF 2305 shows a type of chirp distortion that may result from a lens with spherical aberration. As compared with PSF 2300, PSF 2305 has relatively linear arms near the center. So long as the orientation chirp is complete (invertible to recover the image data), imaging performance will not be degraded. Even if not complete, imaging performance may be acceptable if the orientation chirp is sufficiently invertible to recover images to a desired resolution.

A lens has coma if light passing through different annuli centered on the lens forms annuli on the image sensor whose center varies with annulus radius. As shown in PSF 2310, coma produces an elongated and distorted, but complete spiral. Petzval field curvature is the aberration whereby the lens' focal surface is not planar. As with spherical aberration, coma, Petzval field curvature, and other aberrations can be undone if the orientation chip is sufficiently complete.

Lens aberrations can be beneficial in some embodiments. A PSF out-of-focus to one side of the image plane can cover a pattern area of the same size as a PSF out-of-focus to the other side of the image plane. If two such PSFs are identical, then the camera may not be able to distinguish between them. Lens aberrations can render such PSFs distinguishable, however, such as by producing opposite asymmetries, and can therefore allow cameras in accordance with some embodiments to better distinguish point sources along the axis of incidence.

FIG. 24 depicts a tessellated optical element 2400 to illustrate aspects of phase gratings in accordance with some embodiments. Element 2400 is tessellated with spiral ensembles 2410 of sub-elements 2405—depicted as curvilinear boundaries—that are contiguous across tessellation borders (the hexagonal borders are for illustration, and do not represent physical structure in this example). The sub-elements of each ensemble are arranged such that light converged by element 2400 from a point source and passing through one of ensembles 2410 forms a PSF with spatial modulations representative of the ensemble. In one aspect, the tessellated optical element further converges what would otherwise strike a sensor array as a blurry PSF into a PSF that, while of similar size to the hypothetical PSF, contains high-frequency substructure.

Returning for a moment to the example of FIG. 21D, the blur spot PSF 2160 is assumed to be centered on an ensemble of spiral features to produce the spiral PSF 2165. This is a somewhat special case. Point sources at the same distance from the camera yet in general position will have a PSF containing all sub-elements 2405 of at least one ensemble 2410 collected from neighboring ensembles, with some of them spatially wrapped around. In the example of FIG. 24, a PSF outline 2415 represents the area of a central lobe that is off center with respect to any of the sub-gratings 2410, but that nevertheless covers enough grating features 2405 to produce an invertible orientation chirp. In general, it is beneficial that the wrapping of spatial features between ensembles 2410 not substantially alter the magnitude of the components of the Fourier transform of the resultant orientation chirp. A circle like outline 2415, of sufficient area to encompass one of ensembles 2410, can be swept along a path between neighboring ensembles while, for all intermediate circle positions along the swept path, the swept circle contains optical sub-elements arranged at all the orientations contained in the circle at the start of the path (e.g., all positions produce similar spectra, but with shifting phase).

FIG. 25 depicts how the rich pattern of spatial modulations within orientation chirps produced by grating 2400 of FIG. 24 facilitates improved resolution for out-of-focus PSFs. As in other examples, the preceding digit or digits of each element name indicate the figure in which the element was introduced. Using this convention, elements 24## and 25## refer to features depicted in FIGS. 24 and 25, respectively.

In the top row of FIG. 25, light rays from a point source 2500 pass through a lens (not shown) and onto tessellated grating 2400 of FIG. 24 over the area 2505 encompassed by outline 2415 as blurred PSF 2510. The grating creates orientation chirp 2515, which includes a rich set of spatial modulations as a consequence of the sub-elements 2405 within the area of outline 2415. Chirp 2515 is not a spiral because PSF 2510 is not centered on an ensemble (the pattern of chirp 2515 is shown to match the pattern of sub-elements 2405 within the PSF outline 2415). However, the spatial modulations of chirp 2515 are sufficient that chirp 2515 is invertible.

The second row of FIG. 25 is similar to the first, but includes light rays from adjacent point sources 2520 that illuminate overlapping areas 2525 to produce a pair of overlapping, blurred PSFs 2530. The grating creates a discernible pair of orientation chirps 2535 and 2540, the locations of which can be computationally inverted to a higher resolution than could the smeared PSFs 2530. Chirps 2535 and 2540, shown separately to the right, are slightly different from one another because each PSF 2530 impinges upon a slightly different area of the grating.

The third row of FIG. 25 shows a constellation of nine point sources 2542 that illuminate an area 2545 on the grating with overlapping, blurred PSFs 2550, and the resultant nine orientation chirps collectively labeled 2555. As in the last example, the locations of the point sources corresponding to the orientation chirps 2555 can be resolved with far greater precision than could be accomplished using the PSFs 2550.

FIGS. 26A and 26B depict tessellated gratings 2600 and 2605 in accordance with some embodiments. Grating 2600 is depicted using boundaries between high and low features, whereas grating 2605 depicts relatively high and low features in black and white, respectively.

FIGS. 27A and 27B depict tessellated gratings 2700 and 2705 in accordance with some embodiments. Grating 2700 is depicted using boundaries between high and low features, whereas grating 2705 depicts relatively high and low features in black and white, respectively.

FIG. 28 depicts a tessellated grating 2800 in accordance with one embodiment. Grating 2800 depicts relatively high and low features in black and white, respectively.

FIG. 29 depicts a tessellated grating 2900 in accordance with another embodiment. Grating 2900 depicts relatively high and low features in black and white, respectively.

FIG. 30 depicts a filter array 3000 that can be used in accordance with some embodiments to produce color images using cameras of the type detailed in FIGS. 21A-D. Filter array 3000 includes four color filters, a red filter 3005, two green filters 3010 and 3015, and a blue filter 3020. Each filter is associated with what amounts to an instance of a camera like camera 2100 of FIG. 21A that acts as one of four color channels for the overall camera. For each camera like camera 2100, the wavelength band of interest is limited to the wavelengths passed by the color filter in the optical path.

FIG. 31 depicts a color channel 3100, one of four color channels for the embodiment introduced in connection with FIG. 30. Channel 3100 is similar to camera 2100 of FIGS. 21A-D, so a detailed discussion is omitted. Briefly, channel 3100 includes a color filter, a lens 3105 whose optical properties should be tuned for the light frequencies admitted by the color filter, a grating 3110, and a photodetector array 3115. The red filter 3005 of FIG. 30 is inserted somewhere in the optical path and covering the entire field of view, in this case between lens 3105 and grating 3110. Characteristics of channel 3100, such as the focal length of lens 3105, the spacing X between grating 3110 and array 3115, the spatial frequency range of grating 3110, the depth of grating features and composition of grating 3110, and the geometry of the grating sub-elements may be optimized for the selected color. An image processor (not shown) can combine information collected by the color channels to recover a color image.

The color channels can have fixed lenses with distinct focal lengths, or can have the same lens but distinct spacing between the lens and grating. In cameras with multiple channels of diverse focal lengths, the channel or channels presenting the sharpest focus may be used to capture high-resolution scene information, while the other, relatively defocused channels, can provide color information. Techniques to “paint on” relatively low resolution color information onto a high-resolution image are well known to those of skill in the art.

Cameras with multiple channels, whether for the same or different wavelength bands of interest, provide measures of parallax that can be combined with other information derived from orientation chirps to make a depth map of a scene. Also advantageous, information from multiple channels can be used to disambiguate depth in the case where the object exhibits deceptive structure. For example, a scene with in-focus spiral patterns may appear defocussed to a spiral ensemble. In such cases a cross check to one or more additional channels can resolve ambiguity by selecting which “reality” is most consistent with the image data provided by the disparate channels.

Methods and systems for detecting disparities between images of the same scene have wide applications in e.g. video surveillance and medical diagnosis. An image can be compared with a reference, such as to find a defect, or can be compared with a prior image of the same scene to identify changes. The following discussion details image-change detectors that compare successive images of the same scene to detect motion. However, these detectors and the methods they employ can be put to other uses.

Some imaging applications, such as video surveillance, waste considerable power and memory resources monitoring unchanging scenes. To address this problem, some cameras support a low-power mode in which an image sensor's spatial and temporal resolutions are dramatically reduced. Fewer pixels are sensed, and less frequently, which saves power at the sensor, and the relative paucity of data saves image-processing and transmission power. Image data collected in the low-power mode is used to detect changes, and detected changes can trigger the camera to switch to a higher-performance mode that supports higher spatial and temporal resolutions to verify the change detected in the low-power mode and/or capture an image.

FIG. 32 depicts an image-change detector 3200 that supports a low-power mode that is so efficient that it can be powered using (optionally) an integrated photocell. Detector 3200 can be instantiated on a single semiconductor die, and in this embodiment includes a photodetector array 3205 with overlying grating 1700, an analog-to-digital converter (ADC) 3210 to convert analog information from array 3205 to digital data, logic and memory 3215 to process, store, and communicate the digital signals, and a photovoltaic cell 3220 to power each of the other elements. Whether implemented on a single die or multiple die, the detector 3200 can be packaged opaquely, with a transmissive window overlying the location of the grating 1700 (and the optional photovoltaic cell).

Detector 3200 supports a low-resolution, low-power sentinel mode to sense changes in scene activity, and a higher-resolution mode that captures higher-resolution image data responsive to detected changes. Some embodiments support an additional mode or modes, as detailed below. Although logic and memory 3215 can support the change detection and imaging function, some implementations may have a main function that is different from imaging, with the change detection providing an input to the chip to add situational awareness. In such cases, it may not be necessary that photodetector array 3205 contain enough elements to produce a higher-resolution image.

FIG. 33 depicts array 3205 of FIG. 32 as an array of pixels 3300. Eight nonadjacent pixels 3300 are darkened to indicate a subset that is active in the low-power mode. An orientation chirp 3305 represents a sharply focused response from an exemplary imaged point source as it may appear at the sensor plane. Chirp 3305 is illustrated as dark on a light background for ease of illustration, but would appear as a relatively bright pattern. Chirp 3305 illuminates a set of R pixels within a convex hull 3310, the smallest convex set of S pixels that includes all the illuminated pixels. (Convex hull 3310 may be visualized as the shape formed by a rubber band stretched around chirp 3305.) Chirp 3305 provides a rich set of spatial modulations spanning the areas of hull 3310 that dramatically increases motion sensitivity.

Conventional image sensors resolve a point source as a focused “dot” on a sensor array. If a camera is to detect very small movements, a rich set of active pixels much be maintained even in a low-power mode Imagine, for example, that a point source is resolved as a sharp or blurred dot on array 3205 such that only one or a collection of neighboring pixels is illuminated. In that case, the point source could move considerably relative to the sensor without detection by any of the eight active pixels in this low power mode.

Chirp 3305, the result of illumination by a point source, includes “arms” of changing light intensity that illuminate many more pixels, including nonadjacent ones, than would a resolved spot, and that sweep across a great many pixels 3300 as the point source moves relative to the sensor. Consequently, fewer pixels 3300 need be polled to cover the visual field than with a system employing traditional focusing optics. In this example, movement of the point source that moves chirp 3305 a pixel or two in any direction within the X-Y plane would impact at least one of the eight active pixels 3300, and could thus be sensed. Sensing may involve analog-to-digital conversions of the signals from the same subset of photodiodes at different points in time. In other embodiments, analog sample-and-hold circuits and comparators can be used to signal changes in the imaged field of view. Depending upon the application, such sensed motion could be the information of interest, or could be used to bring detector 3200 out of the low-power mode to take and store one or more sets of relatively high resolution data.

Some embodiments support additional operational modes, or “stages.” In one embodiment, for example, logic and memory 3215 support a three-state machine comprising a sentinel stage, a tentative stage, and a confirmation stage that use progressively larger sets of image data to sense and confirm changes in an imaged scene. In the sentinel stage, n1 pixels are monitored to generate compact image frames, relatively small sets of image data that represent considerably lower image resolution than the maximum available from the sensor. If k1 (≤n1) of the pixel values change by a criterion value θ₁ between successive image frames, then the state machine transitions to the tentative stage.

In the tentative stage, n2 pixels are monitored to generate frames of higher resolution than those generated in the sentinel stage but still less than the full field available from the sensor. If k2 (≤n2) of the pixel values change by a criterion value θ₂ between successive frames, then the state machine transitions to the confirmation stage; otherwise the system reverts to the sentinel stage. If the system is in the confirmation stage, n3 pixels are monitored and if k3 (≤n3) of these pixels change by a criterion value θ₃ between successive frames, then the state machine emits a signal denoting image change detected and remains in the confirmation stage. In this example, the frames captured in the confirmation mode represent the full resolution of the image sensor. Detector 3200 can take some action responsive to detected motion, such as to capture and invert an image, or to alert some external system to the detected motion. From the confirmation stage, detector 3200 can transition back to the tentative or sentinel stage to save power, or responsive to the absence of detected motion.

One benefit of this system is that, because of the grating optics, each photodetector pixel responds to a range of positions in the field of view; thus the number of pixels that needs be monitored is lower (dissipating lower power) than in a traditional lens-based system, in which each pixel responds to a very small range of positions in the field of view. Circuit analyses show that some ADC embodiments can obtain sub-400 nW image change detection, with the power required of ADC 3210 dominating. Address generator circuits for polling subsets of pixels in support of reduced power consumption are well known to those of skill in the art, so a detailed discussion is omitted.

FIG. 34 depicts array 3205 of FIG. 33 with twenty-five nonadjacent pixels 3300 darkened to indicate a subset n2 that is active in an example of a tentative mode in which an increased number of pixels are polled for increased motion sensitivity. Though not apparent in this image, sensitivity can also be changed by e.g. adjusting intensity-change thresholds and frame rates. Should motion be detected in this mode, the imaging device can enter a confirmation stage in which all pixels 3300 are polled. The resultant frames can then be used to confirm motion, and can be used to produce images that can be understood by a human observer. The resultant frames can be stored locally and read later, or can be transmitted to a remote device. The production of images from human observers can likewise be done locally or remotely. In some embodiments confirmed motion can activate and direct a separate, higher-resolution imaging system.

FIG. 35 depicts response 1930 of FIG. 19 as an exemplary chirp that defines a convex hull 3500. To find the convex hull for a given imaging device, response 1930 can be captured by the sensor array. With the brightest pixel(s) serving as a reference, those pixels with at least 10% of that maximum brightness are included in that set of pixel values representative of the response. Convex hull 3500 is the smallest convex set of pixels that includes that set of pixel values. The convex hull is not used for image acquisition or analysis, but affords a measure of response area that can be used to characterize the ratio of active pixels relative to the response and the richness of spatial modulations. In this example, response 1930 includes many and diverse spiral “arms” that collectively provide much more information for detecting motion than would a focused spot.

The examples of FIGS. 33-35 include but one point-spread response, and the response is nearly as large as the pixel array. Detectors in accordance with other embodiments can have more of fewer responses, and each response can occupy a larger or smaller convex hull. For example, an image detector can employ a tessellated optical element like that of FIG. 24 to produce a point-spread response that exhibits a web of features that can be distributed over all or a portion of an underlying array.

FIG. 36 is a flowchart 3600 illustrating a method of detecting apparent motion in accordance with one embodiment of detector 3200 of FIG. 32. Photovoltaic cell 3220 provides sufficient power to support a sentinel mode in ambient light, with enough extra to charge integrated or external energy-storage devices capable of supporting bursts of use in higher-performance modes. In some embodiments detector 3200 includes a separate or integrated RFID chip and associated antenna to allow image data to be retrieved wirelessly. Detector 3200 can support other forms of wired or wireless connections, as will be understood by those of skill in the art. An example of array 3205 with a grating 1700 is detailed in an accompanying document entitled “Ultra-Miniature Low-Power Lensless Image Change Detector,” which is incorporated herein by reference.

However detector 3200 is powered, logic and memory 3215 (hereafter “logic” 3215) automatically enters the low-power sentinel mode on power up (3605). Once in this mode, logic 3215 repeatedly polls n1 pixels, a small subset of the total available in array 3205, to produce small frames of image data (3610/3615). Successive frames are then compared to identify differences.

For any successive pair of frames, per a decision 3620, if some or a combination of corresponding pixel values exhibit intensity changes Δθ that exceed a threshold T1, then logic 3215 enters a tentative mode in which logic 3215 repeatedly polls n2 pixels, wherein n2>n1 but is still a subset of the total available in array 3205, to produce larger frames of image data (3625/3630).

Per decision 3633, device 3200 determines whether some or a combination of corresponding pixel values from the successive frames taken in the tentative mode exhibit intensity changes AO that exceed a threshold T2. Device 3200 can remain in the tentative mode for some number of frames without intensity changes meeting threshold T2, but will eventually return to the sentinel state to save power. Should threshold T2 be met, logic 3215 enters a more power-intensive confirmation mode in which logic 3215 repeatedly polls n3 pixels (3635/3640).

In this example, the value n3 is the total available in array 3205 and thus produces full frames of image data. For any successive pair of full frames, per a decision 3645, if some or a combination of corresponding pixel values exhibit intensity changes Δθ that exceed a third threshold T3, then logic 3215 issues a signal confirming detection of movement (3650). Device 3200 can remain in the confirmation mode for some number of frames without intensity changes meeting threshold T3, but will eventually return to the sentinel mode to save power. In other embodiments device 3200 can transition to the tentative mode before continuing to the sentinel mode. In embodiments that are power limited, device 3200 can enter a standby mode or one of the lower-power modes despite detected motion to allow time to generate or store sufficient power to return to the task of motion detection.

The thresholds T1, T2, and T3 used in the different modes can be different, each tailored for the needs of the different modes. Also, the different modes can be accompanied by changes in e.g. frame rate and the integration time employed by ADC 3210 and/or logic 3215 to acquire image data.

Comparisons of successive sets of image data are not limited to just two sets, or to adjacent sets. For example, frames can be averaged or otherwise combined to reduce the effects of noise, and individual or averaged sets of frames can be compared with earlier individual or averaged sets of frames. Change detection may also be based on comparisons with multiple distinct reference image data. For example, an image sensor experiencing motion due to e.g. wind may produce a large but finite number of distinct reference frames or sub-frames that represent no movement within a scene. The same might be true where wind disturbs elements of scene (e.g., a waving branch). A detector might learn such sets and indicate change only when the most recent set of image data fails to match any in the preceding sets.

Array 3205 and grating 1700 can be created using standard CMOS processes, and its formation is thus compatible with any number of functional blocks. Virtually any integrated circuit that might benefit by inclusion of an imaging device can be adapted to include one. For example, a technology referred to as “smartdust” describes systems in which many small electromechanical systems (MEMS) can be operated on a distributed, wireless computer network to collectively perform various sensing and communication tasks. Smartdust devices can be on the order of a few millimeters on a side, which is easily sufficient to incorporate a sensor of the type detailed herein. In one embodiment, for example, the inventors created a 128×128-pixel sensor that is 200 microns on a side. Image sensors of the type detailed herein can be integrated so inexpensively that they can be incorporated into credit cards and other forms of identification for security purposes, or to facilitate vision systems in the field of microrobotics.

The foregoing detectors support modes that poll different numbers of pixels to provide different tradeoffs between power usage and resolution. Other embodiments can support more, fewer, and different modes. For example, and an image-change detector can have but one mode using all available pixels, but could nevertheless support modes that employ different thresholds, framerates, integration periods, sweep rate, etc.

Some lighting fixtures produce not a steady illumination, but a flicker, often at the frequency of the AC current powering them, or at twice this frequency. Many applications would benefit from being able to reject this flicker, yet stay sensitive to the motion or appearance of other types of light sources. The selective blindness to repeating flicker due to illumination periodicity can be achieved by any of the following four means. First, a bulk photoelement (a photodiode or photovoltaic) without any angle-selective optics over it is sensitive to the overall brightness of the scene. If the integration period of each of the active pixels is not governed by time per se, but rather governed by crossing a threshold in the accumulated photocurrent incident on the bulk photoelement, then the signals observed at each active pixel is scaled by the overall brightness of the room, which will be sensitive to flicker in a way similar to the active pixels. Second, the integration period of the active pixels can be set to an integer multiple of the repetition period of the flickering illumination. Third, the integration periods for any two signals that are to be compared can commence at the same phase of the flickering illumination. Fourth, the signals from each active pixel can be pooled to arrive at an estimate of the overall brightness, and individual signals can first be normalized by this brightness before subsequent normalized frames are compared.

While the subject matter has been described in connection with specific embodiments, other embodiments are also envisioned. For example; while each grating detailed previously may be used in connection with photoreceptors to collect incident light, gratings in accordance with these and other embodiments can be used more generally in imaging devices that project images from photo-emitters rather than or in addition to sensing them; cameras described as using lenses could also employ other types of optical elements (e.g., mirrors); the wavelength band of interest can be broader or narrower than the visible spectrum, may be wholly or partially outside the visible spectrum, and may be discontinuous; and cameras and gratings detailed herein can be adapted for use in multi-aperture or programmable-aperture applications. The wavelength band of interest is the visible spectrum in these examples. Other variations will be evident to those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. § 112. 

What is claimed is:
 1. An image-change detector comprising: a photodetector array of pixels to sense incident light over a wavelength band of interest; a phase grating to induce near-field spatial modulations responsive to a point source, the near-field spatial modulations illuminating a pattern over the photodetector array responsive to the incident light; and logic coupled to the photodetector array to successively poll a subset of the pixels to produce sets of image data, compare the sets of image data, and detect an image change responsive to the comparison of the sets of image data.
 2. The image-change detector of claim 1, wherein the subset of the pixels consists of a first number of the pixels in a low-power mode and a second number of the pixels higher than the first number in a second mode.
 3. The image-change detector of claim 1, wherein the pattern comprises an orientation chirp.
 4. The image-change detector of claim 3, wherein the orientation chirp is computationally invertible.
 5. The image-change detector of claim 1, wherein the near-field spatial modulations are wavelength independent over the wavelength band of interest.
 6. The image-change detector of claim 1, wherein the near-field spatial modulations exhibit radial lines.
 7. The image-change detector of claim 1, further comprising a photovoltaic cell integrated with the photodetector array and the logic to power the photodetector array and the logic.
 8. The image-change detector of claim 1, the pattern illuminating over R of the pixels of the photodetector array and defining a convex hull enveloping an area of S>2R of the pixels of the photodetector array.
 9. The image-change detector of claim 8, wherein the subset of the pixels numbers n1<S/2 in a first mode, the logic to poll a second subset of greater than n1 of the pixels in a second mode.
 10. The image-change detector of claim 9, wherein the logic polls the S pixels in the second mode.
 11. The image-change detector of claim 9, the logic to poll n2 of the pixels in the second mode, where n2<S, wherein the polling the n2 pixels produces successive sets of n2 pixel values, the logic to transition from the second mode to a third mode responsive to intensity changes in the successive sets of n2 pixel values.
 12. The image-change detector of claim 11, the logic to transition from the second mode to the first mode if the intensity changes are below a threshold.
 13. The image-change detector of claim 9, wherein the logic polls the n1 of the pixels at a first frame rate in the first mode and polls the greater than n1 of the pixels at a second frame rate in the second mode.
 14. The image-change detector of claim 9, wherein the logic polls the n1 of the pixels over a first pixel integration time in the first mode and polls the greater than n1 of the pixels at a second pixel integration time in the second mode.
 15. The image-change detector of claim 8, wherein the pattern comprises a spiral.
 16. The image-change detector of claim 8, the logic to poll n1<S/2 of the pixels in the convex hull in a first mode and to poll greater than n1 of the pixels in the convex hull in a second mode.
 17. The image-change detector of claim 1, the phase grating having a minimum separation from the photodetector array of no more than 400 times a maximum wavelength within the wavelength band of interest.
 18. The image-change detector of claim 1, wherein the subset of the pixels includes nonadjacent pixels. 